Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 https://doi.org/10.1186/s10033-021-00550-x Chinese Journal of Mechanical Engineering

REVIEW Open Access Friction‑Induced Nanofabrication: A Review Bingjun Yu and Linmao Qian*

Abstract As the bridge between basic principles and applications of nanotechnology, nanofabrication methods play signifcant role in supporting the development of nanoscale science and engineering, which is changing and improving the production and lifestyle of the human. Photo lithography and other alternative technologies, such as nanoimprinting, electron beam lithography, focused ion beam cutting, and scanning probe lithography, have brought great progress of semiconductor industry, IC manufacturing and micro/nanoelectromechanical system (MEMS/NEMS) devices. However, there remains a lot of challenges, relating to the resolution, cost, speed, and so on, in realizing high-quality products with further development of nanotechnology. None of the existing techniques can satisfy all the needs in nanoscience and nanotechnology at the same time, and it is essential to explore new nanofabrication methods. As a newly developed scanning probe microscope (SPM)-based lithography, friction-induced nanofabrication provides opportunities for maskless, fexible, low-damage, low-cost and environment-friendly processing on a wide variety of materials, including silicon, quartz, glass surfaces, and so on. It has been proved that this fabrication route provides with a broad application prospect in the fabrication of nanoimprint templates, microfuidic devices, and micro/nano optical structures. This paper hereby involved the principals and operations of friction-induced nanofabrication, including friction-induced selective etching, and the applications were reviewed as well for looking ahead at oppor- tunities and challenges with nanotechnology development. The present review will not only enrich the knowledge in nanotribology, but also plays a positive role in promoting SPM-based nanofabrication. Keywords: Scanning probe microscope, Tip-based lithography, Friction-induced nanofabrication, Friction-induced selective etching

1 Introduction fabrication in emerging nanoscale science and engineer- Nanoscience and nanotechnology have brought signif- ing [1]. Depending on whether the template is used or cant improvement of our live quality in wide feld from not, nanolithography techniques are divided into two electronic gadgets to healthcare and medical devices. As types, i.e., masked and maskless lithography. Te masked an important part in triggering the development of nano- lithography is suitable for large-area manufacturing technology, nanofabrication plays a key role in realizing by transferring patterns from templates or molds, and micro/nano-scale structures and device with novel prop- includes photolithography [2], soft lithography [3], and erties in optical, electronic, magnetic or other behaviors. nanoimprint [4]. In contrast, maskless lithography can be Not only micro- and nanolithography has been the main more fexible in fabricating arbitrary patterns by directly driving technology in semiconductor and integrated cir- writing or scanning without any mask, including electron cuit (IC) industry, it also plays an increasingly important beam lithography (EBL) [5], focused ion beam lithogra- role in manufacturing of commercial microelectrome- phy (FIBL) [6, 7], and scanning probe lithography (SPL) chanical system (MEMS) devices as well as prototype [8, 9]. However, none of the existing techniques can sat- isfy all the needs at the same time with the nanotechnol- ogy developing towards diversifcation and depth [10]. *Correspondence: [email protected] With high resolution and fexibility, scanning probe Tribology Research Institute, State Key Laboratory of Traction Power, lithography has attracted great attention worldwide. In Southwest Jiaotong University, Chengdu 610031, China

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1982, Dr. Binnig of IBM Zurich Research Laboratory greatly to the fnal formation of low-damage nanostruc- and his colleagues jointly invented the world’s frst scan- tures either by direct scratching or by post selective ning tunneling microscope (STM) [11]. Tis invention etching, the processes of friction-induced hillock and makes it possible for human beings to observe atoms in friction-induced selective etching can be hereby viewed the atmosphere and at room temperature for the frst as friction-induced nanofabrication. Figure 1 presents the time. STM and subsequent atomic force microscopy development of friction-induced nanofabrication with (AFM) have become powerful tools for nanotechnology some representative events. Compared to the traditional research, and promoting an important enabling technol- nanofabrication methods such as photolithography and ogy, i.e., scanning probe-based nanolithography (SPL) or nanoimprinting, friction-induced nanofabrication has tip-based nanofabrication (TBN). SPL can perform wide excellent properties of low cost, fexibility, and low dam- types of manufacturing activities, from material removal age. During the tip scanning, the trace for tip scanning and modifcation to material deposition and manipula- can be programmed in terms of a demanded pattern, and tion, all in the nanoscale [12]. hence it has no challenges in producing diferent patterns Some pioneering works were realized by scanning at specifed locations without templates [24]. Large-scale probe microscopy (SPM), including STM and AFM. and high-resolution fabrication can be realized through According to diferent processing principles, SPM lithog- multi-probe scanning technology. raphy mainly involves single atom manipulation, mechan- Tis paper will review the principals and operations ical removal, local anodizing, dip-pen printing, thermal of friction-induced nanofabrication, including friction- probe machining, etc [12, 13]. With a SPM tip, surface induced selective etching. Aside from direct nano- atoms could be manipulated controllably via adjusting fabrication with AFM tip scratching, mechanical or the tunneling current or interfacial force between the tip tribochemical mechanisms of material deformation or and the sample [14–18]. Tip-induced local anodic oxida- damages stemming from tip scratching, and the roles of tion (LAO) process can be performed under a given bias diferent structures or phases in selective etching were voltage, and the contact area of a conductive surface is address for fully intercepting the low-damage structure oxidized, forming oxide patterns corresponding to the formation. Some examples for the applications of the designed tip traces [19, 20]. Dip-pen nanolithography fabrication were presented as well for looking ahead at (DPN), a nanofabrication method proposed by Mirkin opportunities and challenges for this nanofabrication et al. in the 1990s, can transfer “ink” molecules to sub- method. strate surface to write specifc nanostructures using AFM tip as a pen, and has promising applications in the felds 2 Direct Nanofabrication with AFM Tip Scratching of physics, chemistry, biology and medical treatment 2.1 Direct Nanofabrication by Mechanical Removal [21, 22]. A SPL method based on the local desorption of Mechanical SPL technique presents high resolution and a glassy organic resist by a heatable probe was proposed low cost for directly fabricating various surface nano- for fabricating nonpatterns at a half pitch down to 15 structures (e.g., nanodots, nanogrooves, and 2D/3D nm without proximity corrections, and complex three- nanostructures) through surface material removal. dimensional structures can be duplicated from a pho- Te lithography is provided with two modes, i.e., static tograph [23]. SPL was also used for fabricating micro/ and dynamic plowing lithography (DPL), which were nanostructures on a fat surface by mechanical removal, respectively operated with contact and tapping modes. but the tip can sufer wear and become blunt. Despite With static plowing lithography, so-called scratching, much progress in AFM nanolithography, there are still various nanostructures can be realized on sample sur- a lot of problems related to the resolution, speed, cost, faces by applying a preset normal load on an AFM tip. smear, and so on, yet to be solved [10]. For instance, using a closed-loop nanoscale precision In the past decade, friction-induced nanofabrication stage integrated with an AFM, 3D nanostructures with was proposed to produce nanostructures on surfaces of complex geometry (e.g., 3D human face shape, nano- wide types of materials, including silicon, gallium arse- line arrays of sine-wave and triangular profles, nanodot nide, glass, quartz, and so on [24, 25]. For the formation arrays of sine-shaped, hemispheric, and concave/convex of hillocks by directly tip scratching, the friction and nanopatterns) were successfully realized on aluminum shear are the necessary conditions for the generation of surface according to predetermined designs in a control- hillocks [26]. Furthermore, the friction-induced hillock lable and reproducible fashion (Figure 2a) [28]. Tese and groove-shaped scratches created under low-normal- nanostructures can hardly be fabricated by bottom-up load scratching can act as a mask resisting against or pro- processes at low cost and with high accuracy. Another moting the etching, i.e., friction-induced selective etching excellent example was the rapid fabrication of com- [27]. Considering that the friction and shear contribute plex 3D nanostructures on poly (methyl methacrylate) Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 3 of 26

 Wang, et al. Appl. Surf. Sci., 454, 2018.  Wang, et al. Sensor. Actuat. B, 288, 2019.  Wu, et al. Phys. Chem. Chem. Phys., 22, 2020.  Peng, et al. Sensor. Actuat. A, 315, 2020. Applications of friction- 2018 —Now induced nanofabrication

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t 2012 —Now selective etching

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Year Figure 1 Development process of friction-induced nanofabrication

9 (PMMA) flm with AFM-based ultrasonic vibration- of the nanodots achieved could be as high as 1.9×10 assisted approach, which was operated in constant height nanodots per ­mm2. Similarity, DPL technique was also control mode [29]. Two methods, i.e., the operations employed to fabricate various structures on graphene under vector mode and raster scan mode, were adopted flm surface (Figure 2c), extending the possibilities for to design stereoscopic features to be machined. Relatively AFM-based manipulation of graphene [32]. Moreover, simple features, such as stair-like nanostructure with fve it was found that the lithography signifcantly depended steps was successfully fabricated in vector mode, while on the applied force. With moderate forces, AFM tip complex nanostructure (e.g., Pyramid 3D structure and only deformed the graphene and generated local strain. the profle of Steve Jobs) were successfully fabricated in In contrast, with sufciently large forces the AFM tip raster scan mode from bitmap images. Tis route can can hook the graphene and then pull it, thus cutting reduce efectively tip wear and enlarge the size of fabri- the graphene along the direction of the tip motion. Fur- cated nanostructures. ther characterization in conjunction with electric force In contrast with static plowing lithography, DPL can microscopy, Kelvin probe force microscopy and conduc- efectively avoid edge irregularities on fabricated nano- tive AFM demonstrated that DPL process cannot change structures, and hence DPL is becoming a promising and local electrical properties of the patterned graphene reliable approach for fabricating complex nanostructures structure. [30]. He et al. [31] applied DPL with an AFM to fabri- cate periodic nanostructures on PMMA flm (Figure 2b). 2.2 Direct Nanofabrication of Protrusive Hillocks In their study, a silicon tip was employed for scratching When scratched by a diamond tip under given condi- PMMA surface under various scanning angles in tapping tions, hillock-like nanostructures can be produced on mode, and high-quality periodic nanodot arrays were monocrystalline silicon surface [33]. Te investigation obtained through optimizing tip traces, and the density of protrusive hillock formation could be traced back to Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 4 of 26

Figure 2 Nanopatterning on material surfaces through mechanical removal by scanning probe: (a) Fabrication of human face nanostructure on Al surface under contact mode [28], (b)–(c) Fabrication of nanostructures on PMMA (Figure 2b) and graphene flm (c) using dynamic plowing lithography based on AFM [31, 32]

the microwear on polymer surface in 1990s [34, 35]. In the generation of hillocks, and this process can be viewed 1990s, Kaneko et al. [33] proposed two kinds of micro- as friction-induced nanofabrication [24]. wear processes of materials. Te frst one is to form directly grooves on material surface, and the other is to 2.2.1 Pressure Threshold for the Formation produce upheaval frstly resulting from plastic deforma- of Friction‑Induced Hillocks on Monocrystalline Silicon tion and tribo-chemistry, followed by the formation of Te scratching by a sharp AFM tip can usually lead to grooves structure. To some extent, friction-induced hill- material remove and thereby form nano-deep grooves, ocks could be a kind of damages in early stage of material which is a typical top-down processing [37]. How- wear. However, Yu et al. [36] found that protrusive hill- ever, protrusive hillocks can also be produced along the ocks can hardly be formed in an indentation test or in a scratching line under some given conditions. Te forma- line-scratch experiment where the scratch distance was tion of grooves or hillocks depends strongly on tip radius too small for diamond tip slipping, but often appeared in and normal load, namely that the contact stress between a long-distance scratch process, as shown in Figure 3. Te AFM tip and sample determines the ultimate appearance sliding and friction appear to be necessary conditions for of hillocks in scratching [24]. Terefore, it is necessary to Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 5 of 26

Figure 3 AFM images of the scars on Si(100) surface after scratching at various displacement amplitudes D [36] detect the threshold load for the transition from protru- oxide thickness, processing atmosphere, and so on, as sion to material removal in the friction process. demonstrated in Figure 5a [24–26, 43]. Under a given Under light normal load contacting, elastic deforma- processing condition, the height of the friction-induced tion can occur on monocrystalline silicon surface [38]. hillocks increased with the applied normal load or Considering the applied normal load and tip radius in the scratching cycle numbers. Under the same loading condi- scratching, the contact pressure threshold can be esti- tion, the hillock produced on Si(100) surface is the high- mated based on Hertz elastic contact theory [24, 39]. Te est, while that produced on Si(111) surface is the lowest contact pressure threshold of hillock formation on silicon [43]. Further analysis deduced that the diference in the was ~11 GPa (Figure 4), which was approximately equal hillock height on various crystal planes may correlate to the calculated value 11.3 GPa by the third strength with surface mechanical properties and bond structures. theory (maximum shear stress theory), and was also close Te formation of friction-induced hillocks was expected to the value 11‒13 GPa [40–42] of Vickers hardness of to be dependent on confgures of dangling bonds, in- monocrystalline silicon. As a result, the determination plane bonds, and back bonds on diferent crystal planes of contact pressure threshold provides the basis for the [44–46]. It is also of great interest to note that the height selection of normal load for a diamond tip with known of friction-induced hillocks decreased with the increase radius for producing friction-induced hillocks. of sliding velocity (10‒1000 μm/s) both in air and in vac- uum [26]. Te sliding velocity-dependent hillock forma- 2.2.2 Processing Parameters‑Dependent Formation tion is related to energy dissipation during tip scratching of Friction‑Induced Hillocks [26], which would be addressed in the following section. Aside from applied normal loads, the fabrication of Especially, in 2010, Yu et al. reported the formation of friction-induced hillocks is also strongly dependent on friction-induced hillocks in vacuum for the frst time, scratching cycle numbers, crystal orientations, surface and the normal load- and scratching numbers-dependent

Figure 4 AFM images (top) and cross-sectional profles (bottom) of the scratches on Si(100) surface after line-scratching under various normal load F [36] (The diamond tip for the test has tip radius of about 0.15 μm, D 100 nm and N 100) n = = Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 6 of 26

Figure 5 (a) Hillock height h as a function of number of scratching cycles N, applied normal load Fn, and sliding velocity v [26]. (b) AFM images and corresponding cross-sectional profles of friction-induced hillocks on silicon and quartz surfaces before and after washing [47]. (c) AFM images of various protrusive nanostructures (i.e., nanodots, surface isolated mesa, nanoline array, and pattern of “TRI”) produced by reciprocating scratching on silicon surface [24]

hillock formation was studied systematically with an tests confrmed that friction-induced hillocks on sili- AFM, as shown in Figure 5a [25]. Based on the vacuum con and quartz surfaces can withstand typical contact test, it is strongly suggested that the friction-induced in dynamic MEMS [47–49]. In view of this, friction- hillock could be composed of deformed silicon, such as induced hillocks presented good mechanical strength amorphous silicon and dislocations, but not only silicon and stability. Friction-induced nanofabrication of protru- oxides. In other words, mechanical interaction should sive hillocks can be easily applied for producing various play a signifcant role in the formation of friction-induced nanostructures on silicon, quartz and glass surfaces. Fig- hillocks. ure 5c displays surface topographies of various protrusive Te mechanical stability of nanostructures directly hillock nanostructures, i.e., nanodots, surface isolated determines their reliability during the service. After the mesa, nanoline array, and pattern of “TRI”, produced by friction-induced hillocks on silicon and quartz surfaces reciprocating scratching of silicon surfaces, confrming were ultrasonically cleaned in deionized water, hardly an excellent fabrication ability. any change in surface topography can be noticed through comparing cross-sectional profles (Figure 5b), demon- 2.2.3 Mechanism for Friction‑Induced Hillock Formation strating that prepared friction-induced hillocks possessed Te formation mechanism of the hillocks has puz- excellent mechanical stability [47]. Also, nanoindentation zled people for a long time, and many eforts have been tests indicated that elastic modulus of friction-induced devoted on revealing the formation mechanism of hillocks on silicon and quartz surfaces was a little lower the friction-induced hillocks from 1990s [50]. Miyake than that of their substrates. Nevertheless, nanoscratch et al. [51, 52] speculated that the generation process of Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 7 of 26

Figure 6 Oxidation-dominated generation of protuberances on silicon surface speculated by Miyake et al. [52]

Figure 7 Comparison of atomic concentration of oxygen in wt% measured by a scanning Auger nanoprobe on the original Si(100) protuberances was ascribed to local atomic destruction surface, on the hillocks created in atmosphere and in vacuum [25] of chemical bonds that occurs with concentrated stress. Specifcally, the reaction of damaged silicon with oxygen and water was enhanced by the sliding, forming silicon also be produced on glass and quartz surfaces, respec- oxide or silicon hydroxide (Figure 6). Since then, it can tively [25]. Such materials are composed of oxidations be accepted that the upheaval formed on silicon sur- or stable compounds, which could be hardly oxidized in face was the results of the oxidation with the participa- scratching, confrming that mechanical interaction, i.e., tion of adsorbed air and water [53–55]. Unlike the past structural deformation, should be responsible for the for- decade, however, Youn and Kang [56] investigated the mation of friction-induced hillocks. efect of single-point diamond machining conditions on Te microstructure of the hillock with a height of ~7 the deformation behavior of monocrystalline silicon, and nm was analyzed by cross-sectional transmission elec- they suggested that the plastic deformation in silicon may tron microscope (XTEM) [24]. As shown in Figure 8, a contribute to the formation of surface hillocks during triangular zone consisting of a bright top layer and a rela- scratching. Meanwhile, considering the formation of fric- tively dark bottom was detected from the cross section, tion-induced hillock in vacuum, the oxidization should which was prepared by focused ion beam (FIB) cutting. not the only contributor to the hillock height. Selected area difraction (SAD) patterns confrmed that To detect any potential chemical reactions during the the bright top case is amorphous silicon with a maximum scratching of Si(100) surface, friction-induced hillocks thickness of ~ 20 nm (Figure 8A; the sharp rings was con- with a height of ~3.5 nm were prepared on Si(100) sur- tributed from protective Pt layer). Beneath the amor- face under area-scanning mode [25]. By scanning X-ray phous layer, a deformed zone with a maximum depth of microprobe detecting, the atomic number ratio of O/Si ~ 80 nm was found with distorted crystal matrix (Fig- increases from 0.60 on the original Si(100) surface to 0.68 ure 8C), which is quite diferent from monocrystalline on the hillocks created in vacuum and 0.97 on the hill- silicon (Figure 8D). Many crystal defects were observed ocks created in atmosphere. Te atomic concentration of in the deformed dark zone, such as slip lines and stacking oxygen was also detected as a function of the depth by faults. a scanning Auger nanoprobe on the hillocks and origi- As shown in the right panel of Figure 8, from EDX nal Si(100) surface, respectively. As shown in Figure 7, analysis on the XTEM sample, both oxygen and silicon the thickness of oxidation layer was 2.0 nm on the hill- were detected form the hillock surface area (A), suggest- ock produced in atmosphere, while the hillock created ing that ­SiO2 may have formed on silicon surface when in vacuum has a 1.7 nm oxidation layer. When deduct- scratched by the AFM tip. However, no obvious oxygen ing 0.5 nm native oxide on original silicon surface, the peak could be detected from the interior of the amor- oxidation layer formed during the scratching should phous zone (B), severe deformed zone (C) and original be 1.5 nm in atmosphere and 1.2 nm in vacuum, which monocrystalline silicon (D). Tese results are consistent were much smaller than the height of the detected hill- with the Auger analysis of oxygen concentration in Fig- ocks (~ 3.5 nm). In addition, friction-induced hillock can ure 7. It is confrmed that oxidation reaction occurred Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 8 of 26

Figure 8 XTEM image showing the cross-sectional structure of a typical hillock formed on Si(100) surface [24] (The left pictures show the SAD patterns from areas: A—amorphous rings for the hillock (sharp rings from Pt), C—deformed and amorphous silicon, and D—substrate silicon (b = 110). The right pictures show the EDX spectra from diferent marked areas of the cross-section)

only at near surface of the hillock and the thickness of oxide layer was much smaller than that of the mechani- cally interacted layer (amorphous silicon and deformed silicon). Terefore, it is logically deduced that even if both tribochemical reaction (i.e., formation of silicon oxide) and mechanical interaction may have contributed to the generation of friction-induced hillocks, the lat- ter (i.e., formation of amorphous structure and crystal defects) should play a dominating role to induce the hill- ock formation on silicon surface. Such a conclusion can be further verifed by the formation of hillocks in vacuum −4 (6.7 × ­10 Pa), on glass and quartz (SiO­ 2) surfaces [24, 48], where it is impossible for oxidizing to occur. For deeper understanding the formation of friction- induced hillocks on monocrystalline silicon, high-reso- lution transmission electron microscope (HRTEM) were performed for detecting cross-sectional microstructures of the hillocks produced under various sliding velocity [26]. As shown in Figure 9a, only a thick amorphous layer was detected on cross section of the hillock produced at a low speed of 10 μm/s in vacuum. By contrast, a thin amorphous layer and severe matrix deformation, includ- ing slip bands and stacking faults, were observed on cross Figure 9 XTEM microstructure of the silicon hillocks created at section of the hillock produced at a high speed of 1000 various sliding speeds in vacuum [26]: (a) XTEM microstructures of μm/s, as shown in Figure 9b, c, and d. Tese slip bands the hillock created at v 10 μm/s, and (b) v 1000 μm/s. The inset = = and stacking faults are aligned on the preferential slip SAD pattern in (b) was taken from the area under the hillock, (c) and planes {111}, which is predicted by molecular dynam- (d) high resolution TEM microstructures of amorphous layer and ics analysis [57]. It is speculated that more energy is deformation zone in (b) Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 9 of 26

speculated to be dissipated in facilitating a-Si formation challenges in wear debris, tool wear, and substrate dam- during scratching at low speed. In contrast, the amorphi- age to some extent. zation is difcult to occur within the short interaction Many scholars were devoted to investigating the feasi- time, and hence most of the input energy is dissipated bility of this fabrication technique and revealing involved in the formation of deformed silicon during high-speed mechanisms. Youn et al. [56] investigated the efect of sliding [47]. single-point diamond machining conditions, e.g., tip ori- As a result, a thicker amorphous layer corresponds entation and normal load, on both deformation behav- with a higher hillock, and mechanical interaction ior and chemical properties of monocrystalline silicon through the generation of amorphous structure is the key through nanoscratch and KOH solution etching. It was contributor to hillock formation on silicon surface. Based demonstrated that the orientation of Berkovich indent on the detection of oxygen concentration by a scanning tip showed a signifcant efect on etching behaviors of Auger nanoprobe in Figure 7, oxidation-induced incre- silicon in KOH solution (Figure 10), and higher normal ment of hillock height during scratching was estimated, load resulted in the increase of etch-mask ability of the and the oxidation contribution to the total height of the mechanically afected layer. Besides, if the sample sur- detected hillocks is less than 23% [25]. Considering that face is non-fat, the tip can adjust to surface variations the scratched area is partly oxidized (Figure 7), later and machines a constant-depth nanostructure. Sung estimation indicated the contribution from oxidation et al. [60] successfully applied SPL technique to fabri- to the height of the hillock is as less as 3.1% in vacuum cate patterns on a tapered 1-hexadecanethiol (HDT)/Ag and 11.3% in air [47]. Moreover, the increase of scratch- surface with a height and width of 2.5 µm and 100 µm, ing cycles can lead to further amorphization or break of respectively (Figure 11a). After being etched in oxygen- Si–Si bonds [56], yielding higher hillocks. In addition, ated cyanide solution for 20 s, the pattern was success- when the normal load increased, more silicon matrix was fully transferred from HDT to the Ag surface because the amorphized and thereby higher friction-induced hillocks HDT can act a mask against the etching (Figure 11b). were produced. Te localized amorphous structure was Also, some special structures, such as slop and a expected to present lower density than crystal silicon, structure with vertical side walls, can be realized by tip and hence piled up to form a hillock [58]. Similarly, the scratching and subsequent selective etching. Park et al. amorphization-dominated interaction may be the main [61] used this technique to fabricate a protrusive hillock contributor for the formation of friction-induced hillocks with a slope (Figure 12a). In their study, a diamond tip on other materials, such as glass and quartz [25, 47, 48]. was used for scratching silicon surface with a given pitch It should be noted that fused silica and glass are typi- in KOH solution, and the generation of the structure cal amorphous materials, and the surface hillocks may was attributed that the scratched regions can act a mask be resulted from by the amorphization of short-range against the etching while unscratched surface continued ordered structures. However, the details for the gen- to dissolve. Moreover, the inclination (α) of the protru- eration of friction-induced hillocks remains to be fur- sive structures was demonstrated to present strong cor- ther clarifed, which can start from the investigation on relation with scratching speed in y direction, but does energy threshold, crystal transition paths, amorphous not tightly correlate with scan pitch, speed in the x direc- transformation, and so on. tion, and normal force. Kawasegi et al. [62] applied this technique to fabricate high-aspect-ratio structures on silicon surface (Figure 12b). Trough scratching along 3 Friction‑Induced Selective Etching on Silicon <112> direction on Si(110) surface and subsequent selec- with Scratch Mask tive etching, a structure with vertical side walls was fab- Te fabrication based on scanning probe scratching ricated, and the width and height of the structures were faces many challenges. For instance, the aspect ratio of dominated by normal load and scratching number. Tis fabricated structures was relatively low, and the scratch- fabrication technique was becoming a promising candi- ing process would easily cause material damage and tip date for the fabrication of Si-based functional structures wear, impacting subsequent applications of fabricated and/or devices. structures. In view of this, combing SPM scratching and wet chemical etching, friction-induced selective etch- 3.1 Mechanisms for Scratch Masking in Friction‑Induced ing [27], which is also called tribo-nanolithography or Selective Etching scanning mechano-chemical probe lithography [59], In past study, researchers speculated that oxygen-rich was developed for fabricating micro/nanostructures on layer in scratched regions can resist etchant etching, material surface. Tis route could meet SPL technical resulting in the formation of protrusive nanostructures on silicon surface [51, 52, 62, 63]. However, it is noted Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 10 of 26

Figure 10 AFM images (top) and corresponding cross-sectional profles (bottom) of nanoscratches created with diferent tip direction (φ 0°, 45°) = [56]: (a) before etching; (b) after 20 wt.% KOH-etching for 18 min

Figure 11 Nanopatterns fabricated on an inclined HDT/Ag surface through a diamond-coated tip scratching and subsequent chemical etching [60] Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 11 of 26

Figure 12 Fabrication of silicon nanostructures through scratching silicon surfaces with a diamond tip followed by subsequent wet etching [61, 62]: (a) Schematic diagram showing the sliding process in KOH solution. The right panel displayed AFM topography of fabricated structures, and the

correlation of inclination (α) of the protrusive structures and scratching speed in the y direction (Vy). (b) Schematic diagram showing the shape of the structure fabricated by the scratching along <112> and <110> directions. The right panel displayed SEM image of high-aspect-ratio structures fabricated by the scratching along <112> direction and subsequent etching in KOH solution

that mechanical scratch-induced subsurface deforma- model proposed by Seidel et al. [65], it is deduced that tion not only consists of oxidation layer, but also con- the selective etching behavior was mainly attributed to tains amorphous silicon and deformed silicon lattice [64]. the diferences on the dangling bond density between Accordingly, the newly formed protrusive nanostructures scratched regions and original silicon surface. When sili- after the etching cannot be attributed completely to the con surface was scratched, the atomic arrangement of anti-etching performance of superfcial oxygen-rich amorphous silicon could become out of order, and the layer in scratched regions. Hence, some researches were average dangling bond density in scratched regions was dedicated to uncovering selective etching mechanism of lower than original Si surface [66]. Accordingly, etching scratched silicon surface in various etchants, e.g., KOH, rate in scratched regions was slow down, resulting in the TMAH and HF/HNO3. formation of hillock nanostructures. Similar performance To clarify selective etching mechanism for scratched and mechanisms were assigned on the friction-induced regions in KOH solution, superfcial oxidation layer of selective etching on silicon surface when TMAH solution a groove-shaped scratch was removed completely by was employed [67]. HF solution, and hillock structures were still observed Te mixture of HF and ­HNO3 solution was isotropic after the HF-treated scratch was further etched by KOH system for processing silicon wafers, and was good com- solution (Figure 13) [27]. Such a result showed that the patibility with on-chip circuitry. However, etching pro- deformed silicon lattice in the scratched area can act a fle was limited by isotropic dissolution of silicon, which mask against KOH etching. Based on the electrochemical impedes its practical applications for the fabrication Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 12 of 26

Figure 13 Anti-etching behavior of the distorted silicon layer on groove structure [27]: (a) A groove produced by scratching on Si(100) surface. (b) AFM image of the scratch after being etched in 10 wt.% HF solution for 15 min to remove surface oxidation layer. (c) AFM image of the scratch after being etched in 20 wt.% KOH IPA solution for 25 min + of nano-sized functional devices [68]. Fortunately, HF/ completely all scratch-induced subsurface defects includ- HNO3 mixture presented excellent selective etching per- ing amorphous and deformed silicon, but HF solution formance on scratched regions and thereby fabricated can only etch amorphous and is powerless for deformed high-quality nanochannels [69]. It is noted that the depth silicon. Accordingly, Wang et al attributed the peculiar of groove-shaped scratch can be deepen after being selective etching phenomenon to the coupling action of etched by HF solution, but its inner surface roughness three factors, i.e., lattice defects, low activation energy was relatively high and the depth of nanochannels was and excess holes [69]. As main subsurface defects, amor- lower than that by HF/HNO3 mixture. In contrast, when phous silicon and dislocations widen lattices and silicon the etchant was replaced by ­HNO3 solution, selective atom spacing, providing a shortcut for etchant molecule etching was hardly detected. Hence, the excellent selec- difusing to reaction interface. Moreover, input mechani- tive etching ability for scratched regions in HF/HNO3 cal energy during scratching process facilitated electrons mixture was attributed to the synergistic efect of HF and transition and emission, leading to a relatively high holes ­HNO3 solution. To further clarify the selective etching density in scratched regions, which can be benefcial for mechanism, cross-sectional microstructures of scratched rapid dissolution of scratched regions. regions before and after being etched by HF and HF/ HNO3 mixture was observed by HRTEM (Figure 14). Te results indicated that HF/HNO3 mixture can remove

Figure 14 HTEM detection on cross section of scratched regions before and after being etched by HF and HF/HNO3 mixture [69] Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 13 of 26

3.2 Efects of Normal Load and Etching Time was attributed to subsurface deformation induced by on Friction‑Induced Selective Etching diamond tip with diferent curvature radii. As a result, Normal load in scratching and etching time were dem- through selecting suitable normal load and etching time, onstrated to show a signifcant efect on the formation various nanostructures with any layout patterns were of nanostructures by friction-induced selective etch- fabricated on silicon surface by friction-induced selective ing. It is found that fabrication width was efectively etching in KOH and TMAH solution (Figure 15). controlled by the normal load, while fabrication height In case of HF/HNO3 mixture etching, the fabrica- almost remained to stable after the scratches produced tion height/width was tightly relevant to normal load under contact pressure beyond 6.3 GPa were etched by in scratching and the time for post etching (Figure 16). KOH solution [27, 56]. Zhou et al. also found that the Specifcally, when low normal load is used for scratch- fabrication height is irrelevant with normal load during ing, selective etching phenomenon was hardly observed, friction-induced selective etching in TMAH solution as which can be ascribed to elastic deformation under the long as the mask was active [67]. Te fabrication height pressure less than yield limit of monocrystalline Si (~7 was both closely related with etching time in the two ani- GPa). Protrusive hillocks were formed from higher load- sotropic etchants, i.e., KOH and TMAH solution, and the induced scratch (2 m·N) after the etching, and it can be diference in etching time-dependent selective etching attributed that amorphous silicon can act a mask in HF/

Figure 15 Fabrication of nanopatterns on Si surface through friction-induced selective etching in (a) KOH and (b) TMAH solution [27, 67]

Figure 16 Normal load-dependent friction-induced selective etching in HF/HNO3 mixture [69] Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 14 of 26

HNO3 mixture etching [70]. However, scratches pro- duced under excessive load can facilitate the etching in a HF/HNO3 mixture, resulting in the formation of deeper grooves, and the groove depth after the etching increased with the increase of applied load. Such results indicated that higher normal load can lead to more serious subsur- face deformation, facilitating rapid selective etching. In addition, the fabrication depth stemming from the etch- ing of higher-load scratches increased with etching time and then remained to be stable, and maximum depth was afected by volume ratio of HF/HNO3 mixture.

3.3 Efect of Temperature on Friction‑Induced Selective b Etching Aside from normal load and etching time, the tempera- ture is an important parameter for establishing a control- lable nanofabrication process by the selective etching. It is found that the fabrication height increased as the tem- perature raised before the collapse of the nanostructures during friction-induced selective etching in KOH solu- tion (Figure 17a), and the selective etching rate was ftted well by Arrhenius equation (Figure 17b) [71]. Moreover, the etching process under diferent temperatures cannot introduce extra contamination based on XPS detections. It is also noted that surface properties, e.g., elastic modu- lus, hardness and contact angle, were obviously afected Figure 17 Temperature-dependent friction-induced selective etching on Si(100) surface [71]: (a) Correlation of fabrication height by etching temperature. Zhou et al found similar tem- and etching time under various etching temperature during perature-dependent selective etching performances in friction-induced selective etching in KOH solution, (b) The variation of TMAH solution, and suggested that relatively low tem- the selective etching rate with reciprocal temperature (1/T) perature, such as 25 °C, can be suitable for the realization of controllable nanofabrication due to the moderate etch- ing rate. On the contrary, the fabrication depth decreased of indentation force, and low indentation force can with etching temperature increased in HF/HNO3 mix- facilitate the formation of sharp nanotips. Te nano- ture [69]. tips height and radius gradually increased and then As a result, even though the temperature increased decreased with etching time, and controllable fabrica- etching selectivity in KOH solution, surface quality was tion was realized at 5‒20 min. Further analysis indi- reduced to some extent. Moreover, HF/HNO3 mixture cated the formation of pyramidal tips is ascribed to presented relatively poor selective etching characteristics anisotropic etching of silicon and etching stop of (111) and high surface roughness at high temperature. Hence, crystal planes in KOH solution, and the mechanism relatively low etching temperature was recommended for was further verifed by the fabricated nanostructures friction-induced selective etching nanofabrication. on Si(110) and Si(111) surfaces.

3.4 Nanoindentation‑Induced Selective Etching 4 Defect‑free Nanofabrication by Friction‑Induced It was well known that nanoindentation can cause sub- Selective Etching on Silicon surface deformation of silicon, and thus the indented Crystal defects induced in fabrication process can regions were expected to act as a barrier layer to resist degrade, to some extent, optical and electrical perfor- the etching. Accordingly, Jin et al. [72] proposed inden- mances of Si-based devices, limiting their lifetime and tation-induced selective etching approach for the fab- reliability [64]. Specifcally, defect-rich surfaces can rication of silicon pyramidal nanotips (Figure 18). It cause rapid recombination of electron-hole pairs which was noted that indentation force and etching time decreases average carrier lifetime, and increase current had an obvious efect on fabrication process. Gener- leakage that may amplify noise [73]. In order to obtain ally, under the same etching conditions, the height of defect-free Si nanostructures, it is necessary to optimize newly formed pyramidal nanotip rises with the increase fabrication process of friction-induced selective etching. Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 15 of 26

Figure 18 Fabrication of pyramid tip arrays by indentation-induced selective etching on silicon surface [72]: (a) & (b) AFM images of indented regions produced under 2, 3, and 4 m N before and after the etching in KOH solution for 10 min, (c) cross-sectional profles of fabricated tips were · obtained from the dotted lines marked in (b)

4.1 Tribochemistry‑Induced Selective Etching Wang et al. [76] developed a UV/ozone-assisted tri- Figure 19 displays the process of tribochemistry- bochemistry-induced selective etching approach for the induced selective etching for fabricating nondestructive fabrication on silicon surface. UV/ozone system was nanostructures [74, 75]. Firstly, using an AFM ­SiO2 tip, used for the preparation of ­SiOx flm on surface, and the native oxidation layer on silicon substrate surface was oxidation flm was demonstrated to be more benefcial removed by reciprocating scratching, where material for subsequent fabrication in contrast with dry, chemi- removal was through tribochemistry reaction at con- cal, and thermal plasma-assisted oxidation. Formation tact interface. Since contact pressure between tip and mechanism of the oxidation flm may be attributed to sample during scratching process was less than yield photo-oxidation reaction in the system, which mainly limit of monocrystalline Si (~7 GPa), no lattice dam- included two processes, i.e., organic decomposition and age occurred in scratched regions. Ten, the scratched oxide densifcation [77]. Similarly, the oxidation flm in surface was immersed in KOH solution. Since native the target area was removed to expose surface Si atom oxidization layer can act a mask against KOH etching, through tribochemistry reaction using an AFM ­SiO2 deeper nano-trench was produced by subsequent selec- tip. Because of the diference in etching rate between tive etching. Moreover, it was demonstrated that the oxidation flm and exposed silicon regions, deeper proposed method can be realized at common humid- nanogroove was formed by subsequent KOH solution ity condition, and one scanning cycle was enough to etching, and fabrication depth was found to be depend- remove ­SiOx mask in area-scanning mode. HRTEM ent dramatically on oxidation time and etching time. observations further revealed that the fabricated nano- In addition, a simple method, i.e., site-controlled for- structures were defect-free. Te proposed approach mation and removal of local oxide mask, was also devel- was proven to be feasible for the fabrication of vari- oped to fabricate nondestructive nanostructures [20]. In ous nondestructive nanostructures, including nano- this approach, a conductive AFM was frstly used for per- trenches array and multilayered pits. forming local anodic oxidation (LAO) on H-passivation silicon surface, followed by immersing in TMAH solution Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 16 of 26

Figure 19 Defect-free nanofabrication by tribochemistry-induced selective etching [75]: (a) Fabrication process of tribochemistry-induced selective etching on silicon surface, (b) HRTEM observation of a defect-free groove with two magnifed images on side panels

for selective etching, and then the fabricated nanostruc- of HF solution, promoting the etching of the scratched tures were dipped in HF solution to remove the oxidation Si/Si3N4 surface. In contrast with traditional friction- layer. Trough further verifying by CAFM detection and induced selective etching (Figure 18b), this method HRTEM observation, nanostructures produced by the (Figure 18a) can fabricate forming nanostructures with approach were proven to be of perfect crystal lattice [20, lesser damage and deeper depths. 78]. 5 Friction‑Induced Selective Etching on Quartz,

4.2 Friction‑Induced Selective Etching of Si3N4 Mask GaAs and Glass Surfaces Figure 20 illustrates a nondestructive nanofabrication Song et al. [80, 81] used friction-induced selective etch- process on silicon surface through friction-induced ing to fabricate nanostructures on quartz surface. Using selective etching of ­Si3N4 mask [79]. Te whole process an AFM diamond tip (R ≈ 350 nm), a scratched area mainly consists of four steps. Firstly, a diamond tip was without obvious wear was produced on quartz under used for scratching Si surface coated with ­Si3N4 flm. area-scanning mode below the critical contact pressure Secondly, the scratched sample was immersed in HF of 5.1 GPa. Te area was found to promote KOH solution solution for selective etching the scratched ­Si3N4 flm etching, resulting in the formation of surface nanostruc- until Si substrate was exposed. Here the scratched Si­ 3N4 tures. Trough systematically investigating the efects flm can be etched of faster than the intact flm around of scan parameters and etching temperature on the for- the scratch. Ten, the HF-etched Si/Si3N4 sample was mation of nanostructures, it was found that the etching dipped in KOH solution for further selective etching. thickness rose with the increase of the scan load and Since the residual Si­ 3N4 flm on non-scratched regions the number of scan cycles, but decreased with the scan can act a mask to resist the etching, exposed Si on the speed. Furthermore, the rise of etching temperature can bottom of the scratch was selectively etched in KOH improve fabrication efciency but hardly change etch- solution and thereby form groove structure. Finally, ing thickness. However, the wear of quartz under higher the residual Si­ 3N4 flm was removed completely by HF normal load did not help to increase the etching thick- solution. Further analysis speculated that microcracks ness. HRTEM detections revealed no visible deformation induced by the scratching can accelerate the difusion under the scratch mask before and after being etched by Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 17 of 26

Figure 20 Fabrication of line-array patterns by friction-induced selective etching of Si­ 3N4 mask and traditional friction-induced selective etching [79]: (a) Friction-induced selective etching of ­Si3N4 mask. Line-array pattern with 2.5 μm in depth was fabricated by the scratching on Si­ 3N4 mask under F 100 m N, post-etching in HF solution for 30 min, and KOH solution for 4 h in sequence, (b) Traditional friction-induced selective etching n = · (Line-array pattern with 0.54 μm in height fabricated by scratching under F 70 mN and post-etching in KOH solution for 1 h) n =

KOH solution. By depositing the gold flm with a thick- contribution for the selective etching. With the proposed ness of ~ 300 nm on the half of amorphous SiO­ 2 and crys- etching, various nanostructures including slopes, hierar- tal quartz surfaces, it was found that there was no etching chical stages and chessboard-like patterns can be fabri- diference between the covered and exposed surfaces on cated on quartz surface (Figure 21). the crystal quartz surface after the etching by KOH solu- Combing diamond tip scratching and chemical etch- tion and the removal of the gold layers, while a distinct ing in a mixture of sulfuric acid solution (volume ratio step was formed on the amorphous SiO­ 2 surface. Tere- ­H2SO4:H2O2:H2O = 1:0.5:100), friction-induced selec- fore, fabrication mechanism could be attributed to the tive etching was realized to produce hillock micro/nano- selective etching of friction-induced amorphous layer structures on GaAs surface (Figure 22a) [82]. Te height on quartz surface, while lattice distortion provided little of protrusive structures increased with the rise of normal

Figure 21 Friction-induced selective etching on quartz surface [80]: (a) A wearless area produced on quartz surface, (b)‒(d) An inclined surface, hierarchical stages, and chessboard-like patterns through the proposed etching Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 18 of 26

Figure 22 Friction-induced selective etching on GaAs surface [82]: (a) Schematic diagram and AFM images showing the fabrication process, (b) Raman detection on original GaAs surface, scratched surface, and post-etching surface (scratched surface after etching) load or etching time. X-ray photoelectron spectroscopy selective etching method was realized to fabricate hill- detection confrmed that only slight tribochemical oxi- ock nanostructures on glass surface (atom concentration dation occurred on GaAs surface during the scratching ratio Al:Na:Si = 0.23:0.07:1) [84]. Te fabrication height (Figure 22b). Raman spectra on scratched GaAs surface increased as the scan load or the number of scan cycles suggested that longitudinal optical (LO) peak became increased. Further analysis based on X-ray photoelectron wider and positive frequency shift was observed (Fig- spectroscopy suggested that a densifed layer would be ure 22c), which can be attributed to structure disorder generated at scanned regions because the angle of Si–O– of GaAs lattice [83]. In view of this, one can suggest Si in glass decreased and the structures of glass was com- that residual compressive stress and lattice densifcation pressed [85]. Te denser ­AlF3 layer can be formed in induced by the scratching may hinder the attack from HF solution on scanned regions, and acts as a mask to the etchant, and hence the etching rate of the scratched prevent the etching from further etching by HF etchant region was slowed down. As a result, the diference of compared to original glass surface [86], resulting in the etching rate between untreated and scratched region formation of protrusive nanostructures. Moreover, it was resulted in the generation of hillock structures on GaAs demonstrated that the increase of scan load was benef- surface in-situ from the scratched area. Trough a home- cial to the formation of ­AlF3. By programming the load- made multi-probe instrument, the capability of this fab- ing mode and scanning traces, various nanostructures, rication method was demonstrated by producing various such as slope, hierarchical steps, and designed patterns, nanostructures on GaAs surface, such as larger-scale lin- can be produced by the proposed etching on glass surface ear array, intersecting parallel, surface mesas, and special (Figure 23). letters. Moreover, combing AFM diamond tip scratching and chemical etching in HF solution, friction-induced

Figure 23 Friction-induced selective etching on glass surface [84]: (a) schematic diagram showing the fabrication process, (b)‒(d) an inclined surface, hierarchical stages, and “TRI” pattern fabricated on glass surface Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 19 of 26

6 Rapid Nanofabrication by Friction‑Induced regions was lowered. Such a mechanism was also applied Selective Etching to indentation-induced selective etching. By program- Te results shown in Section 3.3 demonstrated that high ming tip scratching traces, various patterns with any lay- etching temperature was not ideal for improving fabrica- out and high-quality optical structures, such as circular tion efciency because it can degrade surface properties. gratings, difraction gratings and pyramid-shaped array, Similarly, to change the concentration of etching etchants were rapidly realized on GaAs surface (Figure 24). seems to play limited role in promoting the nanofabri- Electrochemistry-assisted selective etching can also cation efciency by friction-induced selective etching. facilitate the rapid nanofabrication, which was developed Accordingly, some researches were dedicated to explor- to fabricate non-defective microstructures on silicon sur- ing rapid fabrication approach based on friction-induced face [89]. HRTEM observations indicated that both crys- selective etching. tal distortion and amorphous layer in scratched regions UV/ozone system was also introduced into friction- can be etched rapidly whether the etching process was induced selective etching on GaAs surface for improving applied with positive or negative voltage. By investigating the selectivity and reducing surface roughness [87]. It was the efects of fabrication parameters, i.e., applied voltage, found that the height of fabricated protrusive hillocks normal load and etching time, and substrate parameters in UV/ozone-assisted system was obvious higher than including resistivity and crystal orientation on silicon that of traditional chemical etching within same etching microstructures, they found that all parameters except for time. Trough systematically investigating fabrication the resistivity show signifcant efects on the depth and parameters dependence of hillock height, it is found that width of fabricated structures. Moreover, when applied the fabrication height frstly increased linearly and then voltage polarity was changed, the fabricated groove decreased with etching time, and higher normal load in structures exhibited diferent cross-sectional profles. scratching can result in higher fabrication height. Further Further analysis deduced that when the applied voltage analysis suggested the rapid selective etching was attrib- was positive voltage, the curved structure would result in uted to photo-chemical etching of scratched GaAs sur- the enhancement of electrical fled at the groove area and face. Specifcally, less UV light was absorbed at the side hence many holes (h­ +) were accumulated around these of the grooves due to waveguide efects [88], resulting regions, facilitating rapid etching of scratched regions in a relatively dark region. For n-type materials, photo- (Figure 25a) [90, 91]. When the scratched silicon surface electrons transferred to the relatively dark region, where was etched by negative voltage, the h­ + inside the silicon the photo-electrons were consumed in the reduction of and injected by the power source move toward the back HOGaAsOH, and hence the etching rate of scratched side of the silicon (Figure 25b) [92]. Correspondingly,

Figure 24 UV/ozone-assisted fabrication of optical structures on GaAs surface [87]: (a) circular grating, (b) difraction grating, and (c) pyramid-shaped array (All profles at the bottom were obtained from the corresponding dotted lines on the upper AFM images) Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 20 of 26

Figure 25 Etching mechanism of scratched silicon surface when the etching process applied with (a) positive voltage and (b) negative voltage in

HF/H2O2 solution [89]

the concentration of h­ + varied with the depth, forming micro-fabrication apparatus. With a motorized X/Y linear a concentration gradient diference. Te concentration of stage, the scanning can be realized over a 50 mm×50 mm ­h+ on silicon surface is the lowest, resulting in the gen- area with a maximum sliding speed of 10 mm/s. More- eration of deeper groove structures on scratched silicon over, a deformable parallelogram with two beams was surface. applied to precisely control normal load in scratching. In addition, Wu et al. [93] developed a rapid fabrica- After performing indentation tests, fabricated indents tion method for fabricating nondestructive structures array can be used to locate steel micro balls and thereby through site-controlled formation of friction-induced prepare multi-probe array (Figure 26b) [95]. Combining hillocks followed by selective etching in HF/HNO3 mix- probe scanning and post etching in KOH solution, vari- ture. In this study, it was found that friction-induced ous micro-patterns including line arrays and ring-shaped hillocks can act as a mask against the mixtures etching, structures were fabricated on Si(100) surface. and the fabrication process was signifcantly afected by With the friction-induced selective etching, Si-based volume ratio of HF/HNO3 mixture, sliding cycle, normal master gratings were produced for UV-assisted nano- load, and etching time. Further analysis suggested that imprint lithography [96]. Efects of surface cleaning lower average dangling bond density in scanned regions method and diamond tip shape on the fabrication were may retard the breakup of Si-H bonds and polarized Si Si systematically investigated to obtain high-quality grating backbonds [27], leading to lower dissolution rate in HF/ structures. Compared to conventional cleaning method, HNO3 mixture compared to original Si surface. Cross- magnetic stirring assistant cleaning was demonstrated sectional HRTEM observations revealed defect-free to perform better for removing chemical reaction prod- nanostructures fabricated by the anisotropic etching. ucts and recommended for the production of Si-based Also, several nondestructive nanostructures with difer- gratings via the selective etching. It was also noted that ent patterns were realized on Si surfaces by programming sharp and smooth diamond tip was more benefcial for tip scanning traces. Te proposed method is promising fabricating high-quality gratings compared with blunt for the fabrication of controllable nano-sized functional diamond tip. Moreover, the fabrication height/width devices by HF/HNO3 mixture. and micromorphology of grating structures are strongly dependent on the normal load in scratching and the time 7 Applications of Friction‑Induced for post etching, providing a guideline for controllable Nanofabrication fabrication. Using UV nanoimprint lithography technol- Based on friction-induced fabrication fundamentals, Wu ogy, the fabricated master grating structures were excel- et al. [94] designed and fabricated a multi-probe micro- lently replicated on polymer surface. Furthermore, it was fabrication apparatus, which greatly improved the fab- demonstrated through in-situ characterization approach rication efciency. Te apparatus mainly consisted of with SEM and AFM that the structural features of master actuating device, loading system and control system. grating were perfectly replicated, as shown in Figure 27. Figure 26a illustrated main parts of the multi-probe Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 21 of 26

Figure 26 Schematic illustration of (a) mechanical part of the multi-probe fabrication apparatus, and (b) the preparation of multi-probe array [94, 95]

Friction-induced selective etching provide great oppor- characterization showed that the interface was closely tunities in realizing nanochannels [69, 97], which was combined without distinct side leakage, demonstrating widely used in felds covering microfuidics, chemistry, that the device had a good bonding quality (Figure 28e & biology and so forth. Wang et al. [69] designed, fabricated f). Electrical measurements in 1.0 mM KCl indicated that and characterized a nanofuidic device based on friction- nanochannel current exhibited a good linear relationship induced selective etching. Te device mainly consists with applied voltage, and decreased as a function of time, of ports, seals, micro/nanochannels and substrate lay- which can be attributed that the electrolyte at anodic ers, as shown in Figure 28. Te two reservoirs were pro- side of the nanochannel interface was gradually depleted. duced by multi-probe micro-fabrication apparatus in Moreover, the depth and width of the fabricated channels Figure 26 on Si surface. Te seals and microchannel lay- are larger than Debye length of the electrolyte (~ 9.6 nm), ers were fabricated by flling liquid polydimethylsiloxane and hence the electrical double layers could not overlap (PDMS) into a special mold, and dark-feld fuorescent in the nanochannels and afect electro-osmosis [98]. Te Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 22 of 26

Figure 27 UV-assisted nanoimprint lithography of Si-based master grating structures fabricated by friction-induced selective etching [96]

above results indicated that the prepared nanochannels mask efects of diferent lattice deformation and surface had an excellent performance and thereby can meet prac- oxides for resisting or promoting the etching in a given tice application needs for nanofuidic analysis. enchant. With friction-induced etching, defect-free nanostructures were fabricated via controllable prepara- 8 Conclusions and Perspective Remarks tion and tribochemical removal of silicon oxide mask by 8.1 Conclusions tip scratching on silicon surface. Friction-induced selec- Tis paper reviewed the fundamentals and applications tive etching on quartz, GaAs and glass was shown with of friction-induced nanofabrication, including friction- the fabrication mechanisms. To improve the efciency, induced selective etching. Firstly, direct nanofabrication rapid nanofabrication by friction-induced selective etch- was demonstrated by mechanical removal and friction- ing was developed with externally applied UV radiation induced hillocks. Te generation process of friction- system and electric feld. Finally, the applications of fric- induced hillocks on monocrystalline silicon surface was tion-induced nanofabrication were demonstrated by the addressed regarding the infuence of experimental con- building of multi-probe apparatus for large-scale scratch- ditions, including crystal planes, atmosphere, sliding ing, and the fabrication of nanoimprint templates and velocity, etc, and the mechanism was further clarifed nanochannels. as that the hillocks are dominated by amorphous sili- As a newly developed SPL method, friction-induced con rather than oxides, which provides the fundamental nanofabrication has the merits of little pollution to envi- for friction-induced selective etching. When the surface ronment, low processing damages, strong fexibility, and of monocrystalline silicon with scratches is etched by a low cost. Without any template, the nanofabrication can specifc solution, such as KOH, TMAH or HF/HNO3 be realized on silicon, GaAs, quartz, glass surfaces, and solution, protrusive hillock or deeper groove will be pro- so on, and can fnd its broad applications in the pro- duced; the mechanisms were addressed based on the cessing of functionalized surface textures and micro/ nano devices, such as high-quality optical substrates and Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 23 of 26

Figure 28 Design, fabrication, and characterization of a nanofuidic device based on friction-induced selective etching technique [69]

biochips. However, some remarks remain to be noted for signifcant for the controllable fabrication with friction- improving the friction-induced fabrication. induced selective etching.

8.2 Perspective Remarks 8.2.2 Optimization for Aspect Ratio of Fabricated Structures Although friction-induced nanofabrication technique Usually, friction-induced nanofabrication method can has been investigated for years, most existing investiga- be employed for realizing structures with limited aspect tions were focused on fabrication mechanism. In order to ratio. Proper aspect ratio is required for the nanostruc- advance the application of this technique in high perfor- ture to realize devices. For example, a small slope angle mance devices and even in semiconductor industry, more (< 20°) is required for high quality blazed gratings, while studies still need to be conducted in the future. high aspect ratio structures are signifcant for X-ray dif- fractive optics, such as zone plates [99]. Some strategies, 8.2.1 Generation Mechanism of Friction‑Induced Hillocks such as reducing tip radius, applying an electric or mag- To clarify the generation mechanism of friction-induced netic feld during scratching process to change the type hillocks can not only enrich the theoretical knowledge of subsurface deformation, and friction-induced self- of tribology, but also provide further scientifc support assembly flm formation to improve anti-etching abil- for wet etching process. Previous work was focused on ity of scratched regions, may be attempted to improve revealing the contribution of oxidation and mechani- aspect ratio of fabricated structures. Additionally, some cal interaction to the height of friction-induced hillocks methods, such as adding additives into wet etchants or on silicon surface, while specifc formation mechanism employing alternative substrate (e.g., Si(110)) to change was still not clearly understood up to now. Establishing the etching behavior, can be considerate to control the a widely accepted formation mechanism for friction- shape of fabricated structures. induced hillocks on various material surfaces is still to be done in the future. Moreover, uncovering the gen- eration mechanism of friction-induced hillocks is more Yu and Qian Chin. J. Mech. Eng. (2021) 34:32 Page 24 of 26

8.2.3 Development of Large‑Area and High‑Precision Funding Multi‑Probe Nanofabrication Equipment Supported by National Natural Science Foundation of China (Grant Nos. 51775462, 51991373). Usually, a single probe was used to scratch sample sur- faces for friction-induced nanofabrication, resulting in Competing Interests The authors declare no competing fnancial interests. low fabrication efciency. Te lack of fabrication equip- ment with large-area and high-precision ability hindered Received: 5 November 2020 Revised: 4 February 2021 Accepted: 26 seriously widespread application of this method, and February 2021 the corresponding equipment is urgent to be developed. However, when scaling down fabrication dimensionality towards nanoscale, the design, fabrication, installation, References inspection, and control of multi-probe equipment face [1] A Pimpin, W Srituravanich. Review on micro- and nanolithography tech- huge challenges. niques and their applications. 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